In Path of Exile, when Bestiary League was announced, many players thought that catching a variety of beasts and actually completing your Bestiary would be something useful and part of the league mechanic, but the reality is that most beasts except a few red beasts are more or less useless.
There have already been quite a few suggestions, but what I think would benefit the league as a whole the most would be a clear sense of progression in two parts: first, catching and then, crafting.
For catching, there should be a questline that gives us the ability to find better beasts based on how many different beasts we have already caught in the bestiary. For example, at first, Einhar gives us the ability to see and catch “normal” spirit beasts (yellow beasts), and after we have completed 50% of our bestiary, he upgrades that ability and we can start to see and catch (some of) those “legendary” beasts (red beasts).
If we further collect beasts and complete our bestiary, we will in the end receive the ability to see those beasts required for the strongest crafting recipes, like the item splitting or opening a portal to the bestiary bosses.
For crafting, crafting a (rare) recipe repeatedly should make us better at crafting it. For example, crafting an int shield with dex might yield us a random dex tier at first, but if we craft lots of those, we start to improve and the specific skill “crafting an int shield with dex” levels up, removing (for example) lower level tiers.
There could be multiple levels, each removing one lower level tier until only high level tiers are left. There could even be a level where the craftable tier is tier 0, or one tier higher than the tiers that can drop, but it would take lots of crafts to get there (and it would require a certain monster level).
Of course, some crafts are more valuable than others (phys dmg on weapons compared to dex on shields), so every craft should have individual values in terms of crafts required to level it and maximum tier available.
I think beastcrafting and the bestiary have a lot of potential, but I also think that the current criticisms leveled at it are often justified, so I would love to see them work on improving it, but what I believed is that GGG will of course keep working to improve every aspect of the league they can.